Update Rails compatibility.

README.md

@@ -11,7 +11,7 @@ Compatibility
 
 * Supports multiple Ruby versions: Ruby 1.9.3, 2.x, JRuby, and Rubinius.
 * Supports multiple databases: MySQL, PostgreSQL, SQLite, and MongoDB (1.7.0 and higher).
-* Supports Rails 3 and 4. If you need to use it with Rails 2 please see the `rails2` branch (no longer maintained, limited feature set).
+* Supports Rails 3, 4, and 5. If you need to use it with Rails 2 please see the `rails2` branch (no longer maintained, limited feature set).
 * Works very well outside of Rails, you just need to install either the `json` (for MRI) or `json_pure` (for JRuby) gem.